Machine tools

ABSTRACT

A machine tool  10  to fashion objects from blocks of material having a material-remover  16  able to move in at least two degrees of freedom and having the material  18  controlled by processing circuitry. The processing circuitry  18  determines a path along which the material-remover  16  should move, allowing the depth of a cut made by the material remover to vary. The movement of the material-remover  16  is so controlled to improve bulk material removal when compared to prior art machines.

[0001] This invention relates to an improved machine tool, and improvedmethods of running a machine tool.

[0002] Machine tools can be used to fashion objects from blocks ofmaterial by removing excess material in a predetermined manner to arriveat the desired shape. This fashioning generally involves two processes:bulk material removal, or roughing, and fine working of the object. Bulkmaterial removal is designed to remove excess material as quickly and asefficiently as possible to allow the fine working of the object to takeplace. Fine working of the object is a much finer process in whichmaterial removal is slower, more precise, and consequently the surfacefinish is higher.

[0003] To allow material to be removed a path must be planned for thematerial-remover of the machine tool. Traditionally this was performedby the manual control of a skilled operator. In the last few decadescomputer numerically controlled (CNC) machine tools have becomewidespread and a set of instructions, or computer program, is used tocontrol the path of the material-remover of the machine tool in suchmachines.

[0004] The set of instructions governing the path determines theefficiency of the bulk material removal process. It is desired to removeas much material in as little time as possible, but it is imperativethat the material from which the object will be fashioned is notdamaged.

[0005] Various material removal methods are well known and FIGS. 1 and 2show examples of two such methods. FIG. 1 shows perhaps the simplestapproach in which material 1 around the object 2 is removed by rasterscanning the machine tool. In each pass of the machine tool thematerial-remover will remove an amount of material 1, and the line 4within the material 1 shows the centre line of each cut. It will beappreciated that this method is inefficient since the cutting tip doesnot run continuously, but must be stopped and moved around the object 2in order to complete the cut on the opposite side of the object.

[0006]FIG. 2 shows another prior art method of performing the bulkmaterial removal. Again, the line within the material 1 shows the path,or the centre line of a cut, performed by the cutting tip. The cuttingtip makes a series of passes through the material 1 until only theobject 2 remains.

[0007] However, none of the prior art methods discussed herein removebulk material as efficiently as may be desired.

[0008] It is an object of the invention to ameliorate the problems ofthe prior art.

[0009] According to a first aspect of the invention there is provided amachine tool having a material-remover, the material-remover being ableto move in at least two degrees of freedom and at a particular instantbeing arranged to remove an amount of material from material that it isprocessing, movement of said material-remover being under the control ofprocessing circuitry, said processing circuitry determining a path alongwhich the material-remover should move, and in determining said pathallows the depth of a cut made by the material-remover to vary.

[0010] An advantage of such a machine is that it is likely to be moreefficient than bulk material removal than prior art machine tools. Priorart machines are arranged to determine the path of a machine tool aroundthe object such that the material-remover was constrained to removeroughly constant amount of material at any particular instant.

[0011] Prior art cutting machines remove slices of constant width fromthe material until the material has been removed as desired. This is asdiscussed in relation to FIGS. 1 and 2 above. The applicants haverealised that the prior art machines are sub-optimal because the amountof material removed on each cut is constant and that therefore, thesolution for the optimum material-remover path is over constrained.

[0012] The machine tool may be any one of the following machines: amilling machine; a machining centre, a multi-axis machining centre, alathe.

[0013] Conveniently, the material-remover of the machine tool isarranged to rotate about an axis. Such an arrangement is convenientbecause it facilitates material removal.

[0014] Preferably, the processing circuitry is arranged to attempt tomove the material-remover such that the magnitude of its velocity isroughly constant. Such an arrangement is advantageous because it is moreefficient to run a machine tool at its maximum cutting speed rather thancontinuously accelerating and decelerating the material-remover and canresult in an improved life of the material remover.

[0015] However, the processing circuitry may be arranged to vary thespeed of the material-remover for some complex cutting paths. It will beappreciated that the path of the material-remover will have a minimumcutting radius, which is a function of the speed of the material-removerand determined by the maximum axis acceleration of the machine.

[0016] Therefore, the processing circuitry may be arranged to reduce thespeed of the material-remover in order to achieve smaller cutting radiithan are possible at the target material-remover speed.

[0017] A material-remover has a maximum amount of material that it canremove in a single cut. The minimum amount of material that it canremove in a single cut is zero. The processing circuitry may comprise atrack planner arranged to associate one or more tracks around theperimeter of an object to be machined, the or each track comprising alocus of all the possible material remover paths around the object. Anadvantage of such a track is that it provides a convenient startingpoint from which to calculate a material-remover path.

[0018] Alternatively, or additionally, the processing circuitry maycomprise a track planner arranged to associate one or more contoursaround the perimeter of an object to be machined. In such an embodimentit is preferred to select a contour comprising the centre line of thetrack (although any value of contour between the zero and the maximumdepth of cut is possible).

[0019] Conveniently, the track planner is arranged to associate a seriesof such tracks and/or contours around the perimeter of the object to befabricated. As such, a series of tracks and/or contours are built up,each providing an indication of possible material-remover paths. Ofcourse, whilst the first track and/or contour is based upon theperimeter of the object to be fabricated, subsequent tracks and /orcontours are based upon the previous track and/or contour.

[0020] The track planner may produce tracks that are of variable width.

[0021] The processing circuitry may further comprise a node associatorarranged to associate a number of nodes with predetermined points aroundthe track and/or contour that has been calculated.

[0022] Preferably, the node associator is arranged to associate pointswith corners of the track and/or contour. A corner in the track and/orcontour may be defined as any deviation in the track/contour from astraight line that results in a turn tighter than the minimum cuttingradius of the material remover. Therefore, if the track/contourincorporates a curve having a radius greater than the minimum turningradius of the material remover no node may be associated therewith.

[0023] The node associator may be arranged to associate predeterminednodes with the inside of the track. Additionally the node associator maybe arranged to associate other predetermined nodes with the outside tothe track.

[0024] Conveniently, the node associator is arranged to associate nodesassociated with the inside of the track with convex corners of theobject to be machined. The node associator may be arranged to associatenodes associated with the outside of the track with concave corners ofthe object to be machined.

[0025] Conveniently, the processing circuitry further comprises a curveassociator arranged to associate a curve with each of the nodes producedby the node associator. Preferably, the curve associated with the nodeby the curve associator has a radius corresponding to the minimum radiusof a path of the material-remover of the machine tool. Such anarrangement ensures that it is possible to arrange the path for thematerial-remover such that it does not result in the velocity of thematerial remover being reduced because a turn tighter than the minimumcutting radius at its maximum velocity has been specified.

[0026] The curve associator may be arranged to determine whether a curvethat it is associating with a node produced by the node associator isless efficient than a straight line path, and if this is the case toreplace the curve with a straight line path. The skilled person willappreciate that the material remover has momentum, such that smalldeviations in a path may be smoothed out, and that processing complexpaths requires processing power. Therefore, it may be possible to make apath less efficient by introducing curves that attempt to cause thematerial remover to deviate from a straight line by small amounts whilstincreasing the processing required. Therefore, causing the curveassociator to assess the efficiency of the curve may be advantageousbecause it helps to increase the efficiency of the path along which thematerial remover moves.

[0027] The curve associator may be arranged to reduce the radius of oneor more curves if curves centred on opposite side of the track intersectone another to block the track. Reducing the radius of the curves inthis manner results in a reduction in the cutting speed, but may allow apath to be plotted whereas otherwise, no path may have been possible.

[0028] The curve associator may be arranged to associate more than onenode with any one curve. Such an arrangement is advantageous in somecircumstances (particularly when nodes occur close to one another)because it can remove the need for a number of curves and thus thecomplexity of the solution may be reduced.

[0029] Conveniently, the curves are situated at the nodes generated bythe node associator such that the radius of the curve passes close tothe node and preferably, the curve is centred on the bisector of thecorner. It is convenient for the curve associator to associate curveswith the nodes in this manner since this provides a position that iscomputationally simple to find. Calculation problems may occur if thecurve passes inside the node and ensuring that the curve passes close tothe node, rather than touching the node provides a safety margin helpingthe ensure that the curve never passes inside the node.

[0030] The curve associator may alter the position of the curve suchthat it does not lie on the bisector of the corner. This can beadvantageous if it allows a curve of larger radius to be associated withthe node.

[0031] The curve associator may be arranged to associate circles, orportions of circles with the nodes. Circles are convenient since theyare mathematically simple, but any tangent continuous curve willsuffice. It is preferable that the curvature of the curve is continuous,but not essential.

[0032] Preferably, the processing circuitry further comprises a tangentgenerator arranged to associate a path between each of the curvesgenerated by the curve associator, the path being a tangent to each ofthe curves that it contacts. An advantage of the tangent generator isthat the path so produced is the shortest line around the track producedby the track planner without passing inside the curves generated by thecurve associator.

[0033] The processing circuitry is preferably arranged to convert thetangents generated by the tangent generator together with portions ofthe curves provided by the curve associator into a path for thematerial-remover.

[0034] Conveniently, the processing circuitry is arranged to generatepaths that form a closed loop around the object to be fabricated. Suchan arrangement is advantageous because it completely specifies a patharound the object. However, in other embodiments the processingcircuitry may be arranged to generate paths that are open and do notform a closed loop around the object to be fabricated.

[0035] Preferably the processing circuitry is arranged to produce aseries of paths such that an object can fabricated from a block ofmaterial. In some embodiments the processing circuitry is arranged toproduce linking paths for the material remover linking the series ofpaths previously calculated. The linking paths may be curves, which runone path into the next. Such linking paths are advantageous because theyfurther help to improve the efficiency of the material removal.

[0036] According to a second aspect of the invention there is provided amethod of removing material from a block of material surrounding anobject to be fashioned, said method comprising plotting a path for amaterial-remover of a machine tool, said path being optimised byallowing the depth of a cut made by the material-remover to vary.

[0037] An advantage of such a method is that it is likely to be moreefficient at bulk material removal than prior art methods. Prior artmethods are arranged to determine the path of a machine tool around theobject to be fashioned such that the material-remover cuts to asubstantially constant depth.

[0038] Preferably, the method comprises attempting to move thematerial-remover at roughly a constant speed. Such a method may provemore efficient than continuously accelerating and decelerating the speedof the material-remover and may increase the life of thematerial-remover.

[0039] Conveniently, the method allows the speed of the material-removerto be varied if its path includes a turn requiring an accelerationbeyond that which the machine is capable of providing.

[0040] The method may comprise calculating a first track around theperimeter of an object to be machined. Said track may occur between theminimum and maximum amounts of material that can be removed in a singlecut by the material-remover and comprise the locus of all possiblematerial-remover paths between these minimum and maximum amounts.

[0041] Alternatively, or additionally, the method may comprisecalculating a contour around the perimeter of an object to be machined,displaced from the object by a predetermined amount.

[0042] If a contour is calculated it is preferable that it correspondsto the centre line of a track comprising the locus of allmaterial-remover paths around the perimeter of the object. Thus, thecontour corresponds to a path for the material remover corresponding toa cut to a depth of 50% of its maximum in single pass. The contour mayhowever, correspond to a depth of cut for the material-remover at anyvalue from zero to its maximum.

[0043] A plurality of tracks and/or contours may be built up around theperimeter of the object to be fabricated, preferably with eachsuccessive track and/or contour occurring at a distance corresponding tothe maximum depth of material that can be removed by thematerial-remover in a single cut. Therefore, a series of tracks and/orcontours are built up, each providing an indication of the material thatit is possible for the material-remover to remove in a series of passes.

[0044] Preferably the method comprises associating a number of nodeswith predetermined points around the track and/or contour previouslycalculated. Preferably, each node occurs at points coincident withcorners of the track and/or contour. A corner in the track may bedefined as any deviation in the track from a straight line that resultsin a turn tighter than the minimum cutting radius of the material-remover.

[0045] Conveniently, predetermined nodes are associated with the insideof the track previously determined. Additionally predetermined nodes maybe associated with the outside to the track previously determined.

[0046] Nodes associated with the inside of the track may be associatedwith convex corners of the object to be machined. Nodes associated withthe outside of the track may be associated with concave corners of theobject to be machined.

[0047] A curve may be associated with each of the nodes generated by themethod. Preferably, each curve has a radius corresponding to the minimumcutting radius of the material-remover of the machine tool.

[0048] Such a method can be used to ensure that the path for thematerial-remover is such that it does not enter inside the minimumcutting radii associated with a corner of the object to be fabricatedand therefore, cause the material-remover to slow. It will beappreciated that it is possible to reduce the minimum cutting radius byslowing the speed of the material-remover, although it is desirable notto alter the velocity of the material-remover.

[0049] Conveniently, the method arranges the curves at the nodes suchthat the radius of the curve passes close to the node and preferably,the curve is centred on the bisector of the corner. It is convenient forthe curve to be positioned in such a manner since this provides aposition that is computationally simple to find.

[0050] Further, the method comprises plotting a path comprising atangent between each of the curves, together with a portion of one ormore of the curves. An advantage of such tangents is that the path soproduced is the shortest line around the track without passing insidethe curves corresponding to the minimum cutting radii.

[0051] The method may comprise reducing the radii of curves associatedwith the nodes if the curve extends beyond the track. Such a methodhelps to ensure that the path is possible.

[0052] Conveniently, the method generates one or more paths that formsclosed loops around the object to be fabricated. Such an arrangement isadvantageous because it completely specifies a path around the object.However, in other embodiments the method generates one or more pathsthat are open and do not form a closed loop around the object to befabricated.

[0053] According to a third aspect of the invention there is provided amethod of plotting a path for the material-remover of a machine tool,wherein said method comprises allowing the depth of a cut performed bythe material-remover to vary.

[0054] The method of the third aspect of the invention may take any ofthe features from the second aspects of the invention.

[0055] According to a fourth aspect of the invention there is provided acomputer readable medium coded with instructions to cause a computer toperform the method according to the second aspect of the invention.

[0056] According to a fifth aspect of the invention there is provided acomputer readable medium coded with instructions to cause a computer toperform the method according to the third aspect of the invention.

[0057] The computer readable medium of the fourth or fifth aspects ofthe invention may comprise a disk, a ZIP disk, an LS120 disk, a CDROM, aDVD ROM/RAM, any other form of magneto and/or optical storage, or maycomprise an email, an internet download, or any other transmittedsignal.

[0058] There now follows by way of example only a detailed descriptionof an embodiment of present invention with reference to the accompanyingdrawings of which:

[0059]FIGS. 1 and 2 schematically show prior art methods of removingmaterial from a work piece;

[0060]FIG. 3 to 7 illustrate steps in the method provided by theinvention;

[0061]FIG. 8 schematically shows a processing circuitry of theinvention;

[0062]FIG. 9 shows a typical machine tool according to the invention;

[0063]FIGS. 10 and 11 shows examples of situations in which method ofthe invention may need to be modified;

[0064]FIG. 12 shows a further situation in which the method may needmodifying;

[0065]FIGS. 13 and 14 shows possible solutions to the situation of FIG.12;

[0066]FIG. 15 illustrates how the method can be applied to manufacture athree dimensional cylinder;

[0067]FIG. 16 illustrates how the method of the invention can bemodified for a plurality of nodes occurring close to one another;

[0068]FIG. 17 shows two cutting depths that are needed to completelyspecify a cut for a material remover; and

[0069]FIG. 18 shows a series of example paths that have been calculatedfor a material remover.

[0070] The machine tool 10 of FIG. 9 is a 5-axis milling machine, whichcomprises a work receiving area 12 arranged to hold material 14 suchthat the material can have work performed thereon, a material-remover 16for working on the material 14, and processing circuitry 18 forcontrolling the tool 10. The material-remover 16 rotates at high speedabout its axis, as marked by the arrow 15 in the Figure.

[0071] The processing circuitry comprises a display 20 arranged todisplay information, keyboard 22 for inputting information to theprocessing circuitry 18, and a data carrier reader 24 arranged to readmedia such as CD-ROMs, etc. In some embodiments the processing circuitry18 may be a computer such as a PC, etc. interfaced to the machine tool.In other embodiments the processing circuitry may be provided as part ofthe machine tool 10.

[0072] The processing circuitry 18 is capable of receiving code, eithervia the keyboard 22 or via the data reader 24. This code programs theprocessing circuitry to control the material-remover of the machine tool10 to process the material 14 to fabricate the desired object.

[0073] An example object that it is desired to fabricate is seen in theFigures as a block 2. Once the object 2 has been completely specified tothe processing circuitry 18, the processing circuitry proceeds tocalculate a path for the material-remover 16 such that the object can becut from the material 14. This process can be broken into two stages: afirst, roughing, which removes excess material as quickly andefficiently as possible; the second performing final cleaning of theobject 2.

[0074] It will be appreciated that the object 2 shown in the Figures isa two-dimensional and that real objects are in fact three-dimensional.For reasons of clarity the method described below is described inrelation to the two-dimensional object 2. FIGS. 15 and 17 illustrate howthe method could be applied in three dimensions. Firstly, looking atFIG. 17 it will be appreciated that the material remover 16 is itselfthree dimensional and has a maximum amount of material that it canremove in both the X and Y directions from the material 14.

[0075] Turning to FIG. 17 a cylinder 120 has a series of cuts ofmaterial removed therefrom with the material remover 14 passing in acircular manner around the cylinder 120. The leftmost Figure shows acylinder 120 on which no cuts have been performed, the middle Figureshows a cylinder on which a single cut has been performed, and therightmost Figure shows the cylinder after two cuts have been performed.

[0076] These cuts can be continued until the necessary material has beenremoved, effectively providing a number of terraces around the cylinder120.

[0077] To perform the roughing of the material 14 the processingcircuitry calculates a machine tool path in the following manner:

[0078] The material-remover 16 has a maximum depth that it can cut inany one pass (in both the X and Y directions as shown in FIG. 17). Thisvalue will depend on the properties of the material being cut, and alsoon the material-remover, but is a known value and these known values areentered to the processing circuitry 18. Therefore, in one embodiment thefirst step in the process is to plot a track 26 around the perimeter ofthe object 2, whose depth X corresponds to the maximum depth that thematerial-remover 16 can remove in a single pass. This track 26 shows therange of values that the material-remover 16 can remove in one pass (theminimum value is of course zero) and thus provides a locus of all thepossible paths for a material-remover can perform in a single cut aroundthe object. As can be seen in FIG. 7 the processing circuitry comprisesa track planner 100 for calculating this track 26.

[0079] As can be seen from FIGS. 4 and 18 if a single track 26 does notplot a path allowing the material-remover 16 to remove all of thedesired excess material 14 then multiple tracks can be built up aroundthe perimeter of the object to be fabricated. Two extra tracks 27, 29are shown in FIG. 4 each of which allows a path to be calculated for thematerial-remover 16, and so a series of paths can be built up for thematerial-remover 16. Each of the tracks 26, 27, 29 has the same depth Xrepresenting the maximum cutting depth of the material-remover 16.

[0080] The next step is to associate a plurality of nodes with the track26 around the object 2. A node is associated with every corner of theobject 2 and therefore as seen in FIG. 4 six nodes 28, 30, 32, 34, 36,38 are associated with the track 26. A node associated with a convexcorner of the track 26 is placed on the inside of the track 26 (e.g.nodes 28, 30, 32, 36, 38), whereas a node associated with a concavecorner (e.g. node 34) of the track 26 is associated with the outside ofthe track 26. The processing circuitry comprises a node associator 102to associate the nodes with the corners.

[0081] Next, a circle 40, 42, 44, 46, 48, 50, having a radius equal tothe minimum cutting radius of the material-remover 16 is associated witheach of the nodes. This radius is marked on each of the circles 40, 42,44, 46, 48, 50 in FIG. 4. It will be appreciated that thematerial-remover 16 will have a minimum cutting radius governed by themaximum force that the machine tool 10 can provide on any particularaxis. This acceleration will of course be related to the velocity of thematerial-remover 16 (angular acceleration is the product of forceavailable and the reciprocal of the angular momentum). The circumferenceof each circle 40, 42, 44, 46, 48, 50 passes close to the node, and thecentre of the circle lies on a bisector of the corner. The processingcircuitry comprises a curve associator 104 to associate the circles withthe nodes.

[0082] Strictly, placing the centre of the circle on the bisector doesnot yield the optimal result. The optimal placing of the circle centreis such that the trimmed start and end points of each arc are the samedistance from the track wall. However, placing the centre on thebisector, provided the track is narrow (so that the final cutting pathis approximately parallel to the track 26 walls), will give a resultclose to optimum.

[0083] The track 26 cannot be completely blocked by any on the circlesplaced therein. If this occurs then there will be no way to plot thepath 64 along the track 26. If the track 26 is blocked then theradius/positioning of the circles is altered as discussed below.

[0084] Once the circles 40, 42, 44, 46, 48, 50 are placed then a tangent52, 54, 56, 58, 60, 62 to each of the circles is plotted to join thecircles. It may be possible to avoid touching some of the circles whiledoing this, provided that the lines so drawn remain within the track 26.The processing circuitry comprises a tangent generator 106 to generatetangents to link the circles generated by the circle associator 104.

[0085] Once the tangents 52, 54, 56, 58, 60, 62 have been calculatedthen unwanted portions of the circles are deleted leaving a calculatedpath 64 for the material-remover 16.

[0086] The above method for calculating the path 64 provides a solutionto a global optimisation problem in which the adjustment of a singlecorner affects all of the other corners in the path and may be thoughtof as plotting a racing line around the track 26.

[0087] Although the above method for calculating the path 64 for thematerial-remover 16, is generally acceptable there are a number ofsituations where modifications have to be made. FIG. 10a shows a track26 having a corner 66 therein. Running the method described above couldassociate a circle 68 with the corner 66. However, such a circle 68 willnot result in a valid path 64 for the material-remover 16 since atangent plotted for this circle 68 will lie outside the track 26.Therefore, as shown in FIG. 10b the radius of the circle 68 must bereduced such that it fits within the track 26. Thus, the circle 70 isassociated with the corner. The reduced radius of the circle 70 meansthat the speed of the material-remover 16 will need to be reduced forthis corner.

[0088] A further situation in which the method needs to be modified isshown in FIG. 11. This situation occurs when two corners 72, 74 occur inclose proximity to one another. If the standard method above wereapplied then the path 64 calculated as above crosses over itself andgives a self intersecting path as can be seen at region 76 in theFigure. The circles plotted completely block the track 26 so that thereis no way to plot a path between them and again, the radius of circles78, 80 needs to be reduced in this region to generate a suitable path64. Alternatively, as shown in FIG. 16 it is possible to associate asingle circle 122 with a plurality of nodes 124, 126, 128 so that thetrack 26 is not blocked.

[0089] A further example in which the method may be modified is shown inFIGS. 12 to 14 and can be likened to a chicane, comprising twooppositely oriented corners 130, 132 in close proximity. FIG. 12 showsthe corners 130, 132 once circles 134, 136 have been associated with thenodes. It will be clear from overlap of the circles 138 in FIG. 12 thatthe circles 134, 136 completely block the track 26 so that there is noway to plot a tangent between them. Therefore, as before it is necessaryto reduce the diameter of the circles 134, 136 (which will of courseresult in a reduction of the velocity of the material remover 16).

[0090]FIG. 13 shows chicane of FIG. 12 in which the diameter of thecircles 134, 136 have been reduced so that a path can now be plottedbetween the circles. In FIG. 13 the circles of reduced diameter 140, 142are positioned such that their centre lies on the bisector of thecorner.

[0091]FIG. 14 shows a variation on this arrangement that can result inan increased radius for the circles (which is desirable since it willresult in a higher velocity for the material remover in that region).The circles 144, 146 of FIG. 14 no longer have their centres provided onthe bisector of the corners. It is still possible to plot a tangentbetween the two circles 144, 146 and remain within the track 26.

[0092]FIG. 18 shows a series of paths 64 that have been calculatedwithin a block of material 147 around an object 148. Each of the paths64 forms a closed loop around the object 148. Whereas FIGS. 3 to 7 showthe path 64 calculated in conjunction with a track 26 that correspondsto the maximum cutting depth of the material remover, FIG. 18 shows aplurality of paths 150 that correspond to a contour, or a cut of 50%depth (these correspond to the more angular paths of the Figure). Thepaths 150 correspond to the centre line of the tracks 26 and thegeneration of the centre line is an alternative method of plotting thepath 64.

[0093] The innermost contour corresponds to the perimeter of the object148 and has not been changed. Therefore, the final path 64 wouldcorrespond to the perimeter of the object 148, and must therefore befollowed exactly. Looking at the other contours it is possible to seethat sharp corners have been rounded. Thus, the paths 64 allow thevelocity of the material remover 14 to be maintained by varying thedepth of the cut.

[0094]FIG. 18 shows that a series of paths 150 must be planned toproduce the object 148 from the material 147. These paths are shown inthe Figure as a series of independent paths. However, the materialremover must of course pass from one path to the next. A linear movementto the next path may achieve this once a path has been finished.However, in some embodiments one path is linked to the previous by acurve, that effectively links the paths into a continuous single path.The resulting path may approximate to a spiral.

1. A machine tool having a material-remover, said material-removerhaving at least two degrees of freedom of movement, and at an instantbeing arranged to remove an amount of material, up to a depth of cut,from material that is being processed, processing circuitry beingprovided and arranged to control the movement of said material-remover,said processing circuitry determining a path along which saidmaterial-remover should move, and in determining said path saidprocessing circuitry allowing said depth of cut made by thematerial-remover to vary.
 2. The machine tool of claim 1 in which themachine is a milling machine.
 3. The machine tool of claims 1 in whichthe material remover of the machine tool is arranged to rotate about anaxis.
 4. The machine tool of claim 1 in which the processing circuitryis arranged to attempt to move the material remover such that themagnitude of its velocity is roughly constant.
 5. The machine tool ofclaim 1 in which the processing circuitry comprises a track plannerarranged to associate one or more tracks around the perimeter of anobject to be machined, and the or each said track comprising a locus ofall the possible material remover paths around the object.
 6. Themachine tool of any of claim 1 in which the processing circuitrycomprises a track planner arranged to associate one or more contoursaround the perimeter of an object to be machined, the or each saidcontour comprising a locus of all the possible material remover pathsaround the object.
 7. The machine tool of claim 5 in which the trackplanner produces tracks that are of variable width.
 8. The machine toolof claim 5 in which the processing circuitry further comprises a nodeassociator arranged to associate a number of nodes with predeterminedpoints around the track and/or contour that has been calculated.
 9. Themachine tool of claim 8 in which the node associator is arranged toassociate points with corners of the track and/or contour.
 10. Themachine tool of claim 8 in which the node associator is arranged toassociate predetermined nodes with the inside of the track.
 11. Themachine tool of claim 10 in which the node associator is arranged toassociate other predetermined nodes with the outside of the track. 12.The machine tool of claims 8 in which the processing circuitry furthercomprises a curve associator arranged to associate a curve with each ofthe nodes produced by the node associator.
 13. The machine tool of claim12 in which the curve associated with the node by the curve associatorhas a radius corresponding to the minimum radius of a path of thematerial remover of the machine tool.
 14. The machine tool of claim 12in which the curve associator is arranged to reduce the radius of one ormore curves if curves centred on opposite sides of the track intersectone another to block the track.
 15. The machine tool of claim 12 inwhich the curve associator is arranged to associate more than one nodewith any one curve.
 16. The machine tool claim 12 in which the curvesare situated at the nodes generated by the node associator such that theradius of the curve passes close to the node.
 17. The machine tool ofclaim 12 in which the curve association is arranged to associatecircles, or portions of circles with the nodes.
 18. The machine tool ofclaims 12 in which the processing circuitry further comprises a tangentgenerator arranged to associate a path between each of the curves thatit contacts.
 19. The machine tool of claim 18 in which the processingcircuitry is arranged to convert the tangents generated by the tangentgenerator together with portions of the curves provided by the curveassociator into a path for the material remover.
 20. The machine tool ofclaim 1 in which the processing circuitry is arranged to generate pathsthat form a closed loop around the object to be fabricated.
 21. Themachine tool of claim 1 in which the processing circuitry is arranged toproduce a series of paths such that an object can be fabricated from ablock of material.
 22. A method of removing material from a block ofmaterial to fashion an object therefrom, said method comprising plottinga path for a material remover of a machine tool, said path beingoptimised by allowing the depth of a cut made by the material remover tovary.
 23. The method of claim 22 in which an attempt is made to move thematerial remover at roughly a constant speed.
 24. The method of claim 22which further comprises calculating a track around the perimeter of anobject to be machined.
 25. The method of claim 22 which furthercomprises calculating a contour around the perimeter of an object to bemachined, displaced from the object by a predetermined amount.
 26. Themethod of claim 24 in which a plurality of contours and/or tracks arebuilt up around the perimeter of the object to be fabricated, each ofsaid contours and/or tracks providing an indication of the material thatis possible for the material remover to remove in a series of passes.27. The method of claim 24 which further comprises associating a numberof nodes with predetermined points around the track and/or contourpreviously calculated.
 28. The method of claim 27 which furthercomprises associating predetermined nodes with the inside of the trackpreviously determined.
 29. The method of claim 28 which furthercomprises associating predetermined nodes with the outside of the trackpreviously determined.
 30. The method of claim 27 which further compriseassociating a curve with each of the nodes previously generated.
 31. Themethod of claim 30 in which the curve has a radius corresponding to theminimum cutting radius of the material remover of the machine tool. 32.The method of claim 30 in which the curve is arranged at the nodes suchthat the radius of the curve passes close to the node.
 33. The method ofclaim 30 which further comprises plotting a path comprising a tangentbetween each of the curves, together with a portion of one or more ofthe curves.
 34. The method of claims 30 which further comprises reducingthe radii of curves associated with the nodes if the curve extendsbeyond the track.
 35. The method of claims 22 which generates one ormore paths that form closed loops around the object to be fabricated.36. A computer readable medium coded with instructions to cause acomputer to perform the method of claim
 22. 37. A computer readablemedium coded with instructions that when loaded into a machine toolcause it to function as claimed in claim
 1. 38. A machine tool having amaterial-removing means, said material-removing means having at leasttwo degrees of freedom of movement, and at an instant being arranged toremove an amount of material, up to a depth of cut, from material thatis being processed, processing circuitry being provided and arranged tocontrol the movement of said material-removing means, said processingcircuitry determining a path along which said material-removing meansshould move, and in determining said path said processing circuitryallowing said depth of cut made by the material-removing means to vary.39. A machine tool having a material-remover, said material-removerhaving at least two degrees of freedom of movement, and at an instantbeing arranged to remove an amount of material, up to a depth of cut,from material that is being processed, processing circuitry beingprovided and arranged to control the movement of said material-remover,said processing circuitry determining a path along which saidmaterial-remover should move, and in determining said path saidprocessing circuitry allowing said depth of cut made by thematerial-remover to vary the processing circuitry further comprising atrack planner arranged to associate one or more contours around theperimeter of an object to be machined, the or each said contourcomprising a locus of all the possible material remover paths around theobject, the processing circuitry further comprises a node associatorarranged to associate a number of nodes with predetermined points aroundthe track that has been associated, the processing circuitry furthercomprises a curve associator arranged to associate a curve with each ofthe nodes produced by the node associator, the processing circuitryfurther comprises a tangent generator arranged to associate a pathbetween each of the curves generated by the curve associator wherein theprocessing circuitry is arranged to convert the tangents generated bythe tangent generator together with portions of the curves provided bythe curve associator into a path for the material remover.
 40. A methodof removing material from a block of material to fashion an objecttherefrom, said method comprising plotting a path for a materialremoving means of a machine tool, said path being optimised by allowingthe depth of a cut made by the material removing means to vary.